The warning light doesn't tell you how healthy the battery is, it tells you if there are any problems with the charging system. A flickering light means charging is intermittant for some reason, so it would be worth checking the alternator or voltage regulator, which might be on the way out. It's worth also checking that the warning light circuit and bulb are ok - if it lights up brightly and continuously when you switch the ignition on, then goes off when you start the engine, it's ok. Hope that helps.
On a 2004 Mercury Sable : The battery is in the drivers side of the engine compartment at the front
It is possibley your alternator or battery
On a 2001 Mercury Sable : The battery is located in the front of the engine compartment , on the drivers side
check the recalls!!
Because you are about to throw a rod. You need to find a mechanic quick!
It's probably misfiring. You would need to diagnose why.
On a 1997 Mercury Mountaineer : The " CHECK ENGINE " warning light is just to the left of the battery voltage gauge
The most likely cause is a dirty fuel filter that starves the engine.
Clean the battery cables and make sure the wires to the alternator are tight and without corrosion.
I believe it is mounted below the battery tray in the engine compartment
Usually on the lower end of the engine follow the + battery cable to the solenoid and then to the starter
Try disconecting the battery for 5 min.s and reconecting.